(a) Field of the Invention
The present invention relates to a method for reducing line tension and extension in horizontal lifelines used for fall arrest anchorages. Additionally, this invention relates to a method that can be used to determine total energy capacity of a horizontal lifeline system and the safety factors that can be used for design. Additionally, this invention relates to the method used to predict line tension and extension as input loads and span lengths change.
(b) Discussion of Known Art
Horizontal lifelines are sections of cable or other elongated, usually flexible, members that are used as an attachment structure for tethers that are in turn attached to safety harnesses and the like. The safety harness type device is a device worn by an individual working at an area where the risk of falling is a significant risk.
Horizontal lifeline systems are currently used in many applications for fall arrest anchorages in the manufacturing, processing, transportation, and construction and other industries. These horizontal lifelines may be installed as permanent systems for such applications as pipe racks, loading docks, and hangar facilities; portable systems for such applications as construction; and temporary systems for such applications as maintenance or rescue.
The types of line used in these systems may be steel wire rope, synthetic rope, or flat synthetic webbing. A typical installation for a horizontal lifeline system is to suspend a horizontal cable between two anchorages, typically from 20-ft. to 200-ft. apart. The anchorage elevation is typically 5-ft. above the walking/working surface as is required by geometry restrictions imposed by OSHA regulations. When suspended, a horizontal lifeline must be pre-tensioned to keep the line from having too much sag in the center of the span. The angle that the cable makes at each anchorage, measured below horizontal, is referred to as the xe2x80x9cSag Anglexe2x80x9d. When a horizontal lifeline cable is loaded in the center of a span it imposes a tension in the horizontal lifeline. This tension is proportional to the angle of sag. The lower the sag angle, the higher the ratio between the line tension and the load in the center of the span. This ratio is referred to as the load amplification factor. For example, at 0.5xc2x0 of sag the load amplification factor is approximately 50 to 1. At 7xc2x0 of sag the load amplification is approximately 4 to 1. Hence it can be seen that the load amplification increases exponentially with decreases in sag angle. For this reason, most horizontal lifeline installations use only enough pre-tension, or tension load in the lifeline, so that the cable can maintain a sag in the 7xc2x0 range when loaded. This amount of pre-tension is indicated by the manufacturers and is usually in the 175 to 300-lb. Range, depending on span length and cable weight.
Additionally, some manufacturers use energy absorbers in the horizontal lifeline systems. These energy absorbers increase the hysterisis of the system to decrease rebound, absorb some energy, and elongate the horizontal lifeline upon loading to decrease the load amplification. All manufacturers, however, do not pre-tension their lifelines beyond that level required for proper load amplification during a fall. None set pretension requirements above the 300-lb. level.
The present invention generally relates to a new technology referred to as xe2x80x9cCable Tuningxe2x80x9d that can be used to increase the safety of workers using horizontal lifelines. Historically, horizontal lifeline installations were limited by 2 factors acceptable line tension and acceptable total fall distances. Usually to decrease line tension one had to allow a longer fall distance or (more time) to absorb the fall energy. Conversely, if one was limited by fall distance, it required higher allowable line tensions to absorb the energy in a shorter fall distance (or in less time). It has been discovered that by xe2x80x9cCable Tuning,xe2x80x9d using high pre-tension or pre-tension levels far above those necessary for proper sag angle, and combining this with the use of a shock absorber to controllably elongate the horizontal lifeline at a high pre-tension, that line tension and fall distance could both be reduced and, counter to conventional wisdom, both be done at the same time.
The method included analysis of the following components:
a. a shock absorber with integral line tension indicator;
b. a horizontal lifeline cable;
c. end anchorages;
d. a line tensioner;
e. a method to determine input energy;
f. a method to determine shock absorber energy capacity;
g. a method to determine shock absorbing lanyard energy capacity;
h. a method to determine horizontal lifeline energy capacity;
i. a method to determine horizontal lifeline strain under tension.
As can be understood from the above items, another aspect the invention relates to a method for explaining quantitatively how horizontal lifeline rope absorbs energy and a method for calculating its"" total energy capacity.
It should also be understood that while the above and other advantages and results of the present invention will become apparent to those skilled in the art from the following detailed description and accompanying drawings, showing the contemplated novel construction, combinations and elements as herein described, and more particularly defined by the appended claims, it should be clearly understood that changes in the precise embodiments of the herein disclosed invention are meant to be included within the scope of the claims, except insofar as they may be precluded by the prior art.